ALL VALVE RECEIVER by Harry Lythall

Some weeks ago I began work on an all valve (tube) general coverage RX
for the HF bands,  so I have not done so  much work on the homepage or
packet radio.

All valve units I normally build on a 4x12 cm strip of aluminium. This
gives me the opportunity to change and finalise designs of each  stage
before drilling the final chassis. I decided to begin the construction
with the speaker and audio amplifier  stage and work backwards towards
the antenna so the valves transformers and inductors will be therefore
be numbered in reverse order.

Specifications I am aiming for:

Coverage: 550 KHz to 30 MHz in five bands plus one 'special'.
Sensitivity: 2uV minimum
Modes: SSB, CW and AM
Mains supply: 220v AC or 12v DC
IF: 455 KHz
Valves: Whatever is common and available
Construction: the simplest possible
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AF AMPLIFIER
The A.F. power amplifier valve I chose is an E80L tube, mainly because
this was the first one I put my hands on, plus it is 'beefy' enough to
give a couple of watts of clear undistorted speech.   The circuit will
work well using 6CH6 (EL821), EL84  (6BQ5)  or almost any power output
pentode. The output transformer I used is a modified mains transformer.

TUBE-AF.GIF
The AF valves chosen all have an anode current from 30 - 50 mA, and a
Vg1 of about -6 volts.  The cathode resistor must be therefore be (Ia
+ Ig2) / Vg1 or about 200 ohms for Ia = 30 mA. Make this resistor 470
ohms so the valve is run at about 10 - 15 mA. This will only 'tickle'
it so the valve should last as long as you do.      The 47K supplying
current to G2 is merely  a token gesture to help the valve draw a few
more milli-amperes and reduce distortion.
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AF SPEAKER TRANSFORMER
You can buy this from Vintage Electronic Supply, see my homepage.   If
you want to improvise one then you can get quite passable results with
the following method (the one I used).

The valve anode impedance is about 4Kohms,  plus or minus 50%.Matching
4000 to a 3 ohm speaker requires an impedance ratio of about 1333:1 or
a turns ratio of 36:1.   A 9 v 10 watt mains transformer works nicely.
Use a really cheap transformer so  that you can remove the laminations
(expensive ones are laquered so it is d%&n near imposible to take them
apart).    The laminations are normally composed of 'E' and 'I' shaped
plates inserted alternately into the coil former. This is not good for
our purposes as the anode circuit of the valve has a DC component that
could cause the transformer to saturate.     We need to reassemble the
transformer laminations with all the 'E' inserted into the former from
THE SAME SIDE and the 'I's clamped across the open side. The 'I's must
also have a thin sheet of typing paper between them and the 'E's. This
small gap should be sufficient to prevent the transformer saturating.

When the transformer  is connected to  the power amplifier valve there
will be a small amount of mains 'hum' heard in the speaker.      Heavy
smoothing  capacitors cost money,  so connect the  transformer primary
0-volt to the anode of the valve then connect the transformer 230-volt
terminal to the +250vDC supply. The '220-volt' tapping is used to take
the +250 volts to the rest of the receiver.    Any ripple from the PSU
will be heard in the speaker, but any hum comming from the rest of the
receiver will be in anti-phase so it should cancel out.

AF PRE-AMP, DETECTOR and BFO
The BFO, Detector and AF pre-amp uses any double triode, such as ECC81
(12AT7), ECC83 (12AX7) etc. V2a raises the detected signal to a couple
of volts to feed the AF power amplifier.   The signal is detected by a
simple OA91 germanium diode detector.

TUBE-DE.GIF

The BFO signal from V2b  is also fed into the detector stage via a 2pf
capacitor. This capacitor can almost be formed by twisting two bits of
single-strand insulated hook-up wire together. The cathode resistor of
V2b has  been increased to  22K so the current drawn  by this valve is
almost nothing!!! If it fails to oscillate, reduce the value a little.
Here we do not need a BFO that is kicking out a watt or more of power,
so 'birdies' and other unwanted RF signals, inside the receiver can be
minimised by running the BFO at the smallest  current we can  get away
with.

IF AMPLIFIER
The IF amplifier is based upon an EF80 (6BX6), EF91 (6AM6) or any low-
power pentode you can find.   If G3 is not internally connected to the
cathode then this must be done manually.

TUBE-IF.GIF

All coils will be discussed at the end of this article.

MIXER STAGE
My mixer stage is based upon an ECH81 (6AJ8) triode/pentode valve. Any
old low power RF valve having more than three grids works Ok,  as long
as it is rated at 150-250 volts anode voltage (Va).  I did not use the
triode section of my ECH81 because it was an inconvenience to simplify
to have the cathodes of both sections connected internally.

TUBE-RF.GIF

The sections of the tuned circuits  are connected at the points marked
with an '*' via the band switch. A 2-pole 6-way switch is required for
the RF mixer stage.  This switch should have two wafers, the second is
required in the oscillator stage.

As you can see there has been no provision for AGC but you may be able
to add this yourself later. I saw AGC as an unnecesary luxury. I use a
1K0 ohm pot across the antenna & feed the receiver from the pot wiper.
This forms an RF gain control.  If I find the inclination I may add it
to my receiver and post the modified circuit on my homepage.

Coils are listed at the end of this project.
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LOCAL OSCILLATOR
The local  oscillator is based upon an ECC81  (12AT7)  tube, but again
almost any dual triode valve will work in this situation.

TUBE-OS.GIF

In exactly the  same way  as the RF  mixer stage,  the sections of the
tuned circuits are connected at the points marked with an '*', via the
band switch. A 2-pole 6-way switch is required for the LO stage.

The OB2 valve is a cold-cathode voltage regulator for 150 volts. I did
not use one because I am too stingy to spend the money. I used six 24v
Zener diodes in series. I have however plugged them into a B9A (Noval)
valve base so that I can use a cold cathode tube.

The 2K2 resistor in the cathode of the oscillator valve is now changed
along with the coil for the band. This resistor should be selected for
the highest value that  lets the oscillator reliably oscillate accross
the full band.

Coils for the oscillator are listed at the end of this project.

COMPLETE RECEIVER
Here is the circuit diagram of my completed receiver. I have not tried
to show the RF gain control, this is simply a 1K0 carbon potentiometer
accross the antenna, the wiper feeding the receiver antenna socket.

TUBE-RX.GIF

All the valve heaters are connected to 6.3 volts AC. See my article on
getting 250vDC and 6.3vDC.

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COIL WINDING

T2/T3
These are 455KHz valve type 'IF transformers',  available from Vintage
Electronic Supply company Tempe Arizona. See my homepage for component
supliers.  If you wish to make them yourself then wind 440 turns of 32
SWG wire for both the primary and  secondary, the two coils are spaced
about 6mm apart. Two ferrite slugs are inserted into the former.  Each
coil is brought to resonance with about 200pf.

L1
440 turns 440 turns on a 1/4" (6mm) former fitted with a ferrite slug.

L2
550 KHz - 1.65 MHz - 195 turns, tapped at 26 turns.
1.5 MHz - 4.20 MHz - 78 turns, tapped at 12 turns.
4.0 MHz - 11.0 MHz - 29 turns, tapped at 4 turns
10 MHz - 25 MHz - 13 turns, tapped at 2 turns
20 MHz - 32 MHz - 5 turns, tapped at 1 turn
Space for extra band to be used as 'bandspread'. 

L3
550 KHz - 1.65 MHz - 85 turns, tapped at 10 turns.
1.5 MHz - 4.20 MHz - 56 turns, tapped at 8 turns.
4.0 MHz - 11.0 MHz - 23 turns, tapped at 4 turns
10 MHz - 25 MHz - 12 turns, tapped at 3 turns
20 MHz - 32 MHz - 5 turns, tapped at 1.5 turns
Space for extra band to be used as 'bandspread'. 

TEST & ALIGNMENT
AF AMPLIFIER
Touch V1 Grid-1 and V2a Grid terminals with you finger or the tip of a
soldering iron. You should hear a loud buz. Couple your walkman stereo
to the grid of either and you will hear the music.

IF AMPLIFIER & BFO
If you have the luxury of an RF signal generator, then couple it to T2
(or T3) with a temporary 20 turn winding.     You should hear a lovely
beat-note as you tune through 455 KHz.    Couple this to a cheap AM SW
transistor radio and you will have a crude SW SSB(CW receiver. Tune T2
and T3 for maximum signal.

If the  signal is VERY  strong (over 200 mV)  the BFO may be 'pulled'.
This is what happens if a strong signal is fed INTO the BFO oscillator
circuit.    It is aparent by an unclear 'zero-beat'.  When approaching
zero-beat the oscillator suddenly snaps to zero-beat.    In this case,
reduce the 2p capacitor in value until the effect stops.     If the 2p
capacitor is too  small you  will just have a  low AF output  from the
receiver.

RF ALIGNMENT
Set the bandswitch to range 1 (A-0.55 MHz - B-1.65 MHz) or
Set the bandswitch to range 2 (A-1.50 MHz - B-4.20 MHz) or
Set the bandswitch to range 3 (A-4.00 MHz - B-11.0 MHz) or
Set the bandswitch to range 4 (A-10.0 MHz - B-25.0 MHz) or
Set the bandswitch to range 5 (A-20.0 MHz - B-32.0 MHz)

Select the band required (A - B MHz)
Set the tuning capacitor C1 to MAX
Adjust L3 for a receive frequency of A MHz
Adjust L2 for maximum received signal
Set the tuning capacitor C1 to MIN
Adjust L3 for a receive frequency of B MHz
Adjust L2 for maximum received signal
Repeat these steps until no further improvement is gained

Note that band 6 is own choice.    On my set it is 13.5 MHz to 15 MHz.
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PROTOTYPE PERFORMANCE
I did try to get it up to 40 MHz at the top frequency,  but the LO was
to be unstable above about 34 MHz.  If it did oscillate at 38 MHz then
it generated spurious frequencies at 20 MHz. Cutting the top frequency
down to 32 MHz cured this problem.  Tracking on band one is now almost
perfect with the modified coils values given above.

Sensitivity is about as expected (1.5uV) but it does fall off a little
above 20 MHz (4uV in the 28 - 30 MHz band).   The 'Minimum Discernable
Signal is about 0.75uV - ish!. The image response also falls off a bit
above 20 MHz but that is only to be expected with an IF of 455 KHz.

Other than the above comments,  I am quite happy with my new receiver.
Performance is about  comparable with the  Heathkit RG1 I used to have
in the late 60s,  but that is what I was aimimg for.   I hope that you
have as much fun with this receiver as I had, my fun will now continue
as I rebuild it on a decent chassis.

Files:    TUBERX.TXT
          TUBE-AF.GIF
          TUBE-DE.GIF
          TUBE-OS.GIF
          TUBE-RF.GIF
          TUBE-RX.GIF (the complete re3ceiver)

Have fun.        Harry Lythall - SM0VPO @ SM0VPO.SL0ZS.SOLNA.AB.SWE.EU
                 Ron Lythall   - G0TLA @ GB7NND.#23.GBR.EU
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These files are available for automatic download from two PMS's on the
packet network.  All files are compressed into a single ZIP file.  You
may request them as follows:

Packet command:   SP 7PSERV @ G0TLA.GB7NND.#23.GBR.EU
Message title :   QRP/TUBE-RX/TUBE-RX.ZIP @ (insert your BBS address)
Message title :   /EX (or Ctrl+Z)

                ------------- or -------------

Packet command:   SP 7PSERV @ SM0VPO.SL0ZS.SOLNA.AB.SWE.EU
Message title :   QRP/TUBE-RX/TUBE-RX.ZIP @ (insert your BBS address)
Message title :   /EX (or Ctrl+Z)
----------------------------------------------------------------------
Files are also available via the INTERNET at the following addresses:

http://user.tninet.se/~acz732k/
http://hem2.passagen.se/sm0vpo
